Overall, the use of ovens, furnaces, treatment chambers and the like into which a reagent gas is introduced as part of a treatment step are known (in the description below, the term “oven” should also be understood broadly as being applicable to furnaces and other treatment chambers of this kind). An example is the method of chemical vapor infiltration in which a precursor reagent gas is introduced into an oven having porous elements placed therein (for example and without limitation, such as porous preforms for brake disks).
In general, a conventional oven comprises an outer oven casing enclosing a work space or reaction chamber into which objects or elements for treatment are placed, a system for causing the reagent gas to flow into and out from the oven, and a heater system for heating at least an internal portion of the reaction chamber.
In known manner, the reagent gas is forced to infiltrate the porous structure of the porous elements. The reagent gas may comprise a hydrocarbon gas such as propane.
In a known example, a reagent gas is introduced into an inside volume defined by a stack of annular preforms for brake disks that are substantially in alignment in a reaction chamber of an oven. Generally, the gas is forced to travel from the inside volume of the stack towards the outside of the stack by diffusing through the porous (e.g. fibrous) structure of the preforms and/or by flowing through gaps between adjacent stacked preforms, as defined by spacers or the like.
At least the inside of the reaction chamber is heated by the heater system. Thus, because of the relatively high temperature of the brake disk preforms, the reagent gas is subjected to pyrolysis and leaves a decomposition product that is deposited on the inside surfaces of the porous structure. By way of example, with a hydrocarbon gas, the decomposition product is pyrolytic carbon, thus obtaining a carbon-containing composite material (such as a carbon-carbon material).
In general, ovens of this type are constituted by a plurality of components that are assembled together by welding, bolting, or the like, so as to define the various units of the assembled structure.
Nevertheless, various structural defects or anomalies are commonplace in the structure of an oven. For example, the component parts may be poorly aligned while the oven is being constructed. In other circumstances, intermittent defects can appear, such as poor alignment between parts due to thermal expansion differences while the oven is in use. This happens for example when different materials having different coefficients of thermal expansion are used in the same assembly. In general, structural defects at those sites lead to clearances, gaps, or the like occurring between the parts, through which external air (which might contain contaminants, for example) can penetrate into the oven, and through which the reagent gas (which amongst other things is usually flammable) can escape.
The location(s) through which the gas is introduced into the oven can present a particular problem, at least concerning the reagent gas which might escape from the gas flow path instead of being conveyed usefully into the zone of the oven where the brake disk preforms or the like are located.